Mueller leaks what he wants the media to report. The fact that the media is not reporting the name, means the media don’t know anymore than we do who the person is. So anybody going apoplectic because of some perceived body language is just being silly.
If the Media does know the name, and it was damaging to President Trump, then it would already have been leaked.
If the Media knew the name and it was damaging to the Democrats, they'd delay releasing the name as long as possible.
So it seems likely to me that the Media doesn't know the name yet...